.idx and .sub subtitle files only work if both are present, but the code
was overwriting the first-inserted element to subtitlesByName, as it was
keyed by the basename without extension.
Make subtitlesByName point to a slice of nodes instead.
Seems to be some corner cases that are not being handled, so taking a different
approach that should be a little more robust.
Also, changing resources to be served under a subpath: We've been serving
media at /res?path=%2Fdir%2Ffilename.mp4; change that to be just /r/dir/filename.mp4.
It's cleaner, easier to reason about, and a necessary first step towards just
serving the resources via httplib anyway.
